This invention relates to chemical compositions and, in particular, to chemical compositions which can be used as sources of water in thermally developable photographic film units.
Thermally developable photographic film units are known. In such film units, image formation is effected by simply heating the film unit subsequent to exposure. External processing compositions or baths are normally not employed during processing inasmuch as the reagents necessary to effect image formation are initially contained within the film unit structure and the application of heat is alone sufficient to initiate image formation.
Thermally developable film units heretofore known include those which comprise at least one silver halide emulsion layer and, in one or more layers, a thermally induced water-releasing material, i.e., a material capable of supplying water to the internal environment of the film unit when the film unit is heated. Generally, the water provided by such water-releasing materials is employed to activate one or more silver halide developing agents initially present in the film unit, thereby promoting development of the silver halide. Image formation is effected as a function of development. For example, silver halide reduced to silver in areas of development may be used as the image-forming material, or unreduced silver halide in areas where development has not occurred may be transferred to a receiving layer and reduced therein to provide the desired image.
In addition to activating a silver halide developing agent, the water provided by the water-releasing material may perform other functions depending on the image formation process intended to be operational in the film unit. For example, the water may function to activate a stabilizing agent as disclosed, for example, in U.S. Pat. No. 3,248,219; to promote colloid transfer of silver halide as disclosed, for example, in U.S. Pat. No. 3,080,320; or to facilitate diffusion transfer of soluble silver halide as disclosed, for example, in U.S. Patent Defensive Publication No. T878,008, published Sept. 8, 1970.
Materials which have been disclosed to be useful as thermally induced water-releasing materials in thermally developable film units include water-in-wax emulsions as disclosed in the previously referenced U.S. Patent Defensive Publication No. T878,008; hydrated salts such as sodium citrate pentahydrate, sodium acetate trihydrate, calcium acetate dihydrate, trisodium phosphate dodecahydrate, and hydrated sodium sulfate as disclosed, for example, in previously referenced U.S. Pat. No. 3,080,230; substances which ensure a high residual moisture content such as glycol, glycerol, and sorbitol as disclosed, for example, in U.S. Pat. No. 3,080,230; alkali metal salts of polystyrol sulfonic acid, as disclosed in German Pat. No. 1,175,075; and water-releasing agents such as Glauber salt (sodium sulfate decahydrate), borax, sodium triphosphate, sodium metasilicate, sodium alginate, and sugars as disclosed, for example, in previously referenced U.S. Pat. No. 3,248,219.